A basic problem in understanding multistage carcinogenesis is the dissection of the mechanisms by which tumor promoters can eventually derepress the expression of genes responsible for the malignant phenotype in cells "initiated" by exposure to small doses of carcinogens. Our previous work indicates that one parameter which may play a role in determining stable changes in gene activity is the pattern of DNA methylation and that the tumor promoter 12-0-tetradecanoyl phorbol-13-acetate (TPA) influences changes in DNA methylation. We have also found that the antileukemic drug 5-azacytidine causes human leukemic cells (HL-60) to mature to relatively normal phenotypes under conditions in which it inhibits DNA methylation and have been able to demonstrate that, once incorporated into DNA, it specifically inhibits DNA methyltransferase through tight binding of the enzyme to 5-azacytosine rich regions of DNA. On this basis, we intend to: (1)\use 5-azacytosine substituted DNA as a tool to study the specificity of DNA methyltransferases from HL-60 cells, HL-60 cells that have lost the ability to differentiate after exposure to TPA (HL-60T) and from normal and malignant leukocytes; (2)\determine, by restriction enzyme mapping, how 5-azacytidine treatment of HL-60 cells affects methylation of genes whose activity may be critical in maintaining malignancy and to compare the patterns of methylation with those of the same genes in HL-60T cells and normal and malignant leukocytes; and (3)\determine whether tumor promoters have a direct effect on DNA methylation in HL-60 cells and whether they are capable of modifying the influence of 5-azacytidine treatment on DNA methylation and differentiation.